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The mammalian toll is even higher, concluded researchers from the Smithsonian Conservation Biology Institute and the U.S. Fish and Wildlife Service, ranging between 6.9 billion and 20.7 billion annually.

The analysis, published in the journal Nature Communications, suggests feral and owned cats pose a far greater threat than previously thought. One study in 2011 estimated cats in the United States kill roughly half a billion birds annually.

Peter Marra, the paper's senior author and a research scientist at the Smithsonian Conservation Biology Institute, said he and his colleagues "pulled together all the best estimates" from 90 different studies to reach their estimate, taking into account the difference in behavior between owned and unowned cats.

"I don't think there's ever been an attempt like this," Marra said in a telephone interview, adding the new estimate is "conservative."

Researchers estimate one pet cat kills between one and 34 birds a year, while a feral cat kills between 23 and 46 birds a year. As a result, the new study provides a wide range of the total bird death count. "It's not a single number," Marra said.

George Fenwick, president of American Bird Conservancy, said in a statement that the findings should serve as "a wake-up call for cat owners and communities to get serious about this problem before even more ecological damage occurs."

"The very high credibility of this study should finally put to rest the misguided notions that outdoor cats represent some harmless, new component to the natural environment," Fenwick said. "The carnage that outdoor cats inflict is staggering and can no longer be ignored or dismissed."

Cats pose the greatest danger to birds and mammals living on islands, because there are fewer opportunities for these animals to escape. Cats are responsible for helping drive 33 species of birds, mammals and reptiles to extinction on islands, including the Stephens Island wren from New Zealand in the late 1800s, according to the International Union for Conservation of Nature.

Scientists have a hard time measuring the impact of cats on small mammals in the United States because they lack precise population counts for these species, Marra said.

"We don't know how many Eastern cottontail rabbits are out there, and we don't know how many chipmunks are out there," he said.

By contrast, researchers estimate the United States is home to between 15 billion and 20 billion adult land birds. Cats kill about 10 percent of them each year, according to the analysis.

Marra and two other scientists, the Smithsonian Institute's Scott Loss and Tom Will from Fish and Wildlife, conducted their analysis as part of a broader study of humans' impact on bird mortality. Roughly 150,000 to 400,000 birds in the United States die in wind turbines, according to recent estimates, while between 10 million and 1 billon birds die annually after colliding into glass.

The fact that humans can take action to prevent some of these deaths -- such as adopting policies to reduce feral cat populations and altering how wind turbines are designed -- should provide some hope, Marra said.

"These are things that are reversible once we understand them," he said. "That's the important thing here."

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